Pipe sizing - Replacing a one pipe system with flow and return

[Fwds2andback]

Hi,
I'm a first time poster with a skill set akin to that of a competent DIY'er I guess. I'm planning on replacing a old in efficient one pipe system with a flow and return. This will be done on our recently purchased bungalow which has an oil fired boiler. Whilst I'm confident a 22mm flow and return with 15mm T's for the 15 radiators will be sufficient in terms of heat carrying capacity, I'm concerned that the flow resistance posed by the long pipe runs will be excessive. I was therefore wondering if the entire flow and return down the centre of the house should be run in 28mm? The boiler is approximately 8m 50 from the spine of the house where a 14m flow and 14m return will run. Hopefully the plan below shows the layout in sufficient detail.

I'd be grateful of any advice this forum could offer, thanks!

 

[ShaunCorbs]

Plastic or copper piping ?

 

[Fwds2andback]

Plastic or copper piping ?

Thanks for replying so quickly - copper.

 

[ShaunCorbs]

Have you calculated the heat loss / load of all the rads ?

 

[Fwds2andback]

Have you calculated the heat loss / load of all the rads ?

Yep, I found it to be around 24 kW using the "Whole house boiler sizing method" but will double check that against the other kW/BTU calculators that are online.

 

[ShaunCorbs]

Will be fine on 22mm but if you want to future proof a bit 28mm would be best and tee off in 22mm when there a run of 2 or more rads

 

[Fwds2andback]

Will be fine on 22mm but if you want to future proof a bit 28mm would be best and tee off in 22mm when there a run of 2 or more rads

Thanks very much for the advice. When you say future-proof, I take it that is to provide capacity if I want to add another couple of rads?

 

[ShaunCorbs]

Yes as at time of install cost is marginally more as it’s the same time to install

 

[Knappers]

I think 28mm is overkill.

 

[Fwds2andback]

I think 28mm is overkill.

Hi,
Thanks, I was worried about the long runs of 14 from the boiler to the spine of the house then the flow and return circuit affecting the flow resistance across some 100m of pipe and 15 radiators. As I've no experience of putting in a system this large before I'd be really interested in why you think it's overkill? Thanks in advance.

 

[Knappers]

Hi,
Thanks, I was worried about the long runs of 14 from the boiler to the spine of the house then the flow and return circuit affecting the flow resistance across some 100m of pipe and 15 radiators. As I've no experience of putting in a system this large before I'd be really interested in why you think it's overkill? Thanks in advance.

OK I was abit lazy and just had a quick look.
Didn't soak in your 24kw heating requirement so yes 22mm is on the limit for MFR and with 100m of pipework your possibly at the higher end of head pressure for a standard pump.
It's fine I think (I haven't worked out your pressure losses) on 22mm but as said 28mm would perhaps be a better choice for main branches.

 

[Fwds2andback]

OK I was abit lazy and just had a quick look.
Didn't soak in your 24kw heating requirement so yes 22mm is on the limit for MFR and with 100m of pipework your possibly at the higher end of head pressure for a standard pump.
It's fine I think (I haven't worked out your pressure losses) on 22mm but as said 28mm would perhaps be a better choice for main branches.

Thanks very much knappers, appreciate you taking the time. A couple of last questions I had were:
1. Would taking the flow and return from the boiler to the main spine of the house in 28mm, then reducing the F&R to 22mm be a sensible compromise?
2. Is there any online calculator or guidance that I could use to work out the MFR?
3. I've now calculated each rooms heat requirements using an online calculator, rather than using the "whole house boiler sizing method". This suggests a heating requirement of 15kW for the CH and 2kW for the HW. Therefore it would seem wise to split the difference between the 2 results of 17kW and 24kW and call it a heat requirement of 21kW. Would this difference influence choosing 28mm over 22mm for the main branches?

Thanks again for help, really appreciated.

 

[Knappers]

15/17kw seems like a much more normal requirement for number of rads. If your HW cylinder is next to the boiler then your heating pipework doesn't need to include the requirement for HW.
We're healthily back in the realms of 22mm pipe.

Flow rate(l/s) = kW/(4.2×temp differential)

Temp diff should be 20 or less (I'd use 15)
If you know your flowrates then you can workout pressure losses based on pipesize (or vice versa) for each branch and trunk, with a chart or calculator using velocity of between 0.75-1.5 m/s.
(Pressure Drop Online-Calculator - http://www.pressure-drop.com/Online-Calculator/)

Have fun!

 

[Knappers]

Or you could just do it in 22mm

 

[Knappers]

Incase anyone is interested.
For UFH pipe sizing i'd use a temp differential of 10 or less